Institute of Fundamental Technological Research

Stethoscopes are used to transmit body sounds related to various physiological processes to ears of a physician, providing basic or supportive information for eventual diagnosis. Unfavorably, the dominant frequency components of most of the auscultation signals are localized close to the lower frequency limits of the human auditory system, restricting the achievable selectivity and specificity. The present study introduces an approach that aims at overcoming the existing limitations. A signal processing scheme utilizing knock rejection, dynamic compressor, and pseudo-stereo synthesizer blocks is described, along with hardware implementation and results of the initial subjective evaluation.

Vibrations of a diaphragm of an acoustic stethoscope in contact with a body of an auscultated patient are the source of the sound transmitted to the ears of a physician performing examination. Mechanical properties of a diaphragm can thus significantly affect the parameters of the transmitted bioacoustic signals. However, the exact relation remains mostly unclear, as the underlying phenomena involve complex effects of acoustic coupling between the diaphragm and the body of a patient. The present study introduces a detailed methodology for determining vibroacoustic behavior of a diaphragm of a stethoscope during a heart auscultation. A laser Doppler vibrometer is used to measure the velocity of vibrations at various points on the surface of a diaphragm during the examination. Synchronized recordings of electrocardiography signals are used for segmentation. Representative data sets are selected and analyzed for various kinds of diaphragms. The results show significant differences in vibration velocity levels and their distribution across the surfaces of the considered structures, but no significant filtering effects.

acoustic medical diagnostics, stethoscopes, diaphragms, auscultation

Background: The area of application of electronic stethoscopes in medical diagnostics covers the scope of usability of the acoustic stethoscopes, from which they have evolved and which they could potentially replace. However, the principle of operation of these two groups of diagnostic devices is substantially different. Thus, an important question arises, regarding the differences in parameters of the transmitted sound and their potential diagnostic consequences in clinical practice.
Methods: In order to answer this question, heart auscultation signals are recorded using various stethoscopes and divided into short fragments based on the analysis of the synchronized recordings of electrocardiogram signals. Next, a dedicated algorithm is used to extract representative datasets for each case, which are then analyzed for their acoustic parameters. Four different electronic stethoscopes were investigated, together with an acoustic stethoscope as a reference point.
Results: The determined acoustic characteristics of the considered stethoscopes differ significantly between each other.
Conclusions: The differences in sound transmitted by various stethoscope models may translate into significant differences in quality of the obtained diagnosis. It is also pointed out, that the terminology and application guidelines regarding the electronic stethoscopes are misleading and should be changed.

Stethoscope, Auscultation, Electronic stethoscope, Acoustic diagnostics

The present study introduces a detailed methodology which can be applied for objective evaluation and comparison of the acoustic parameters of medical stethoscopes using auscultation sounds as test signals. The described approach allows taking into account the acoustic coupling between the body of an auscultated patient and the chest piece of a stethoscope. Information obtained from additional, synchronized electrocardiography measurements is used to extract short, specific fragments of recordings, defined as acoustic events. Analysis of the spectral characteristics of many acoustic events allows us to compare the acoustic properties of various stethoscopes and to estimate the measurement uncertainty. The exemplary results of the comparative evaluation of acoustic properties of bell and diaphragm-type chest pieces of a single stethoscope are presented. The results show that the frequency characteristics of the signals obtained using both examined chest pieces under the conditions of the performed examinations are very similar.

Purpose The force with which the diaphragm chestpiece of a stethoscope is pressed against the body of a patient during an auscultation examination introduces the initial stress and deformation to the diaphragm and the underlying tissues, thus altering the acoustic parameters of the sound transmission path. If the examination is performed by an experienced physician, he will intuitively adjust the amount of the force in order to achieve the optimal sound quality. However, in case of becoming increasingly popular autodiagnosis and telemedicine auscultation devices with no such feedback mechanisms, the question arises regarding the influence of the possible force mismatch on the parameters of the recorded signal. Design The present study describes the results of the experimental investigations on the relation between pressure applied to the chestpiece of a stethoscope and parameters of the transmitted bioacoustic signals. The experiments were carried out using various stethoscopes connected to a force measurement system, which allowed to maintain fixed pressure during auscultation examinations. The signals were recorded during examinations of different volunteers, at various auscultation sites. Results The obtained results reveal strong individual and auscultation-site variability. Conclusions It is concluded that the underlying tissue deformation is the primary factor that alters the parameters of the recorded signals.

The paper presents the developed algorithm which implements the indirect variational boundary element method (IVBEM) for computation of the free-field acoustic radiation characteristics of vibrating rectangle-shaped plate structures with arbitrary boundary conditions. In order to significantly reduce the computational time and cost, the algorithm takes advantage of simple geometry of the considered problem and symmetries between the elements. The procedure of determining the distribution of acoustic pressure is illustrated on the example of thin, rectangular plate with a part of one edge clamped and all other edges free. The eigenfrequencies and the corresponding vibrational mode shapes of the plate are computed using the finite element method (FEM). The results of the numerical simulations are compared to the results of the experiments carried out in an anechoic chamber, proving good agreement between the predictions and the observations. The reliability of simulations and high computational efficiency make the developed algorithm a useful tool in analysis of the acoustic radiation characteristics of vibrating plate structures.

Acoustic radiation, Indirect variational Boundary Element Method, Plate structures

Recently, a new class of ceramic foams with porosity levels up to 90% has been developed as a result of the association of the gelcasting process and aeration of the ceramic suspension. This paper presents and discusses original results advertising sound absorbing capabilities of such foams. The authors manufactured three types of alumina foams in order to investigate three porosity levels, namely: 72, 88, and 90%. The microstructure of foams was examined and typical dimensions and average sizes of cells (pores) and cell-linking windows were found for each porosity case. Then, the acoustic absorption coefficient was measured in a wide frequency range for several samples of various thickness cut out from the foams. The results were discussed and compared with the acoustic absorption of typical polyurethane foams proving that the alumina foams with high porosity of 88-90% have excellent sound absorbing properties competitive with the quality of sound absorbing PU foams of higher porosity.

Sound absorption, Porous materials, Alumina foams

The paper focuses on some issues regarding the utilization of small rectangle-shaped piezoelectric transducers as both sensors and actuators in active vibration and vibroacoustic control systems of beam, plate and panelled structures with arbitrary (non-homogeneous) boundary conditions. A new form of description of a simple proportional active control system with multiple independent feedback loops is proposed. The modal sensitivity functions of sensors and the modal selectivity functions of actuators are introduced to describe their ability for sensing and exciting specific structural modes of the structures. Basing on the assumed form of cost function and the derived equations of control system the influence of the modal characteristics of transducers on the stability of the system and on the performance of the active control is analyzed. The results of analytical solutions and numerical simulations are compared with the results of the experiments carried out on various beam and plate structures made up of aluminium or composite materials including the actual materials used in aviation, proving usefulness of the presented approach.

Vibroacoustics, Piezoelectric transducers, Modal sensitivity, Vibroacoustic control systems

The paper describes briefly some main aspects of the active feedback control system that has been developed and constructed for reduction of vibroacustic emission of vibrating plate structures with arbitrary boundary conditions. Relations between the forms and frequency of the vibrations induced by an external harmonic excitation and the distribution of the generated acoustic pressure field are investigated using the developed numerical model based on the Indirect Variational Boundary Element Method. The aim of the control is to minimize the sound pressure level in a given point of the ambient space. The system uses small, rectangle-shaped piezoelectric transducers as both sensors and actuators. The transducers are connected in a number of independent feedback loops, and the feedback gains are the control parameters which are optimized using the developed optimal control algorithm. The constructed active system has been tested for the stability and control performance during experimental research performed in an anechoic chamber. Results of experiments are presented in the paper, proving a high level of noise reduction and a good agreement with numerical predictions.

Vibrating plate structures, Active feedback control system, Vibrational modes

The paper presents results of the theoretical and numerical investigation on acoustic radiation of vibrating plate structures submerged in water. The current state of the art on the considered issues is briefly reviewed. Then, the method for determining eigenmode shape functions and eigenfrequencies of plate vibrating in water that has been used in presented study is introduced. The constitutive equations for solid domain and the pressure acoustic equation for liquid domain are coupled via boundary conditions and solved numerically using the finite element method. Structural mode shapes and eigenfrequencies computed for plate submerged in water are compared to analogous results obtained for air and for the in vacuo case. It is ssumed, that the plate is rectangle shaped and that it is placed in an infinite rigid baffle. Three-dimensional near- and far-field acoustic radiation characteristics for the plate vibrating in water are introduced. Possibilities of implementation of the active control system for reduction of the hydroacoustic emission are briefly discussed.

Hydroacoustics, Plate structures, Hydrocoustic radiation

Vibroacoustic control of thin beam, plate and panelled structures with arbitrary boundary conditions is investigated. The study focuses on determining optimal locations of piezoelectric sensors and actuators on the surfaces of structures under vibroacoustic control. The work consists of three parts. In the first part, the undertaken assumptions and some governing equations are briefly introduced. Then, in the second part of the study, the piezo-transducers' locations which ensure optimal sensing/actuating capabilities for specific vibration modes are determined, basing on the derived analytical formulas and on some results of numerical simulations, as well as on the actuator/sensor equations given in the first part of the study. The relevant modes are selected by taking into account that the main purpose is to minimise the acoustic field generated by the vibrating structure. The third part of the work discusses some experimental investigations aimed for the verification of the results obtained theoretically. Some technical aspects of creating the composite structures for active control systems are briefly described in appendix.

Vibroacoustic panels, Vibroacoustic control, Piezoelectric transducers, Optimal placement

W artykule przedstawiono wyniki badań nad aktywną redukcją transmisji wibroakustycznej przez strukturę płytową. Zakres przedstawionych w pracy zagadnień obejmuje zarówno opis teoretyczny rozpatrywanych zjawisk i układów, jak i rezultaty symulacji numerycznych oraz badań doświadczalnych. Rozważanym obiektem jest płyta aluminiowa o grubości 2mm z przyklejonymi na jednej z jej powierzchni elementami piezoelektrycznymi. Część tych elementów pełni funkcję sensorów, pozostałe zaś stanowią aktywatory, za pomocą których realizowane jest sterowanie aktywne. Kontroler pracuje w układzie sprzężenia zwrotnego, na jego wejście podawany jest wzmocniony i odwrócony w fazie sygnał napięciowy z sensorów, będący jednocześnie sygnałem błędu. Algorytm sterowania realizowany jest w oparciu o klasyczny regulator proporcjonalno-całkująco-różniczkujący (PID), dla różnych konfiguracji połączeń poszczególnych członów. Fizyczna realizacja kontrolera wykonana została w formie układu analogowego bazującego na niskoszumnych wzmacniaczach operacyjnych.

Aktywna kontrola wibroakustyczna, Regulator PID, Aktywna redukcja drgań

Most of the auscultation sounds do not reveal any significant single-frequency components, and their acoustic energy is concentrated in the low frequency region—up to about 100 Hz, falling even below the threshold of hearing. Such character is determined not only by the vibroacoustic behavior of sources, but mostly by high damping introduced by the sound transmission path through tissues underlying the skin surface. The contained diagnostic information is very subtle, and thus it can be easily masked by internal or external noise sources. Not all of those corrupting signals can be efficiently blocked, hence frequency filtering is the most obvious solution for improving the diagnostic capabilities. Many various filtering strategies and techniques were developed and implemented in both acoustic and electronic stethoscopes, however they are based primarily on (not always correct) intuition and subjective evaluation, without implementation of any accurate and objective measurement means or optimization algorithms. The present study introduces various signal to noise ratio (SNR) measures, applicable for different examination cases. Frequency filtering optimization strategies, for maximizing the values of the introduced coefficients for different heart and lung auscultation sounds, are presented.

Active vibroacoustic control of beam and plate structures with arbitrary boundary conditions is considered. The goal is to develop a method of minimizing sound radiation efficiency of such structures. Primary sound field arise as a result of vibrations, due to external disturbances. It is assumed that the control system is compact - it does not contain any additional, ambient microphones. Piezoelectric transducers, mounted on the surface of the controled object, are used as sensors and actuators. Accurate numerical model of the considered structure is needed to determine optimal parameters of the control system. Theoretical background and the results of numerical and experimental research are briefly introduced.

Active vibroacoustic control, Plate structures, Beams, Piezoelectric transducers